As an alternative to a group of astronauts (4)
I believe the answer is:
nasa
'a group of astronauts' is the definition.
'as an alternative' is the wordplay.
'alternative' is an anagram indicator.
'as'+'an'='asan'
'asan' is an anagram of 'NASA'.
'to' is the link.
(Other definitions for nasa that I've seen before include "rocket launcher" , "US agency responsible for space flight" , "US headquarters for space exploration" .)